The present invention relates to a hybrid industrial vehicle, and more particularly, to a hybrid industrial vehicle that charges a battery with a generator motor connected to an engine, executes travel control by driving a travel motor using power of the battery or the power generated by the generator motor, and executes cargo handing control by driving a cargo handling pump using the engine and the generator motor.
A known hybrid forklift combines a so-called series hybrid system for traveling and a parallel hybrid system for cargo handling. In such a hybrid forklift, the engine and a generator motor, which is coupled to the engine, drives a cargo handling pump. Further, the engine and the generator motor are controlled to charge and discharge a battery and supply power to, a travel motor.
Generally, in an industrial vehicle such as a forklift that handles cargoes, the depression amount of an accelerator determines the vehicle speed. This allows for the same vehicle speed to be obtained with the same depression amount of the accelerator regardless of the cargo handled by the industrial vehicle increasing or decreasing the load applied thereto.
A hybrid forklift controls changes in the vehicle speed by providing the travel motor with acceleration instructions and deceleration instructions. The power of the travel motor required to control the vehicle speed changes in accordance with the acceleration or deceleration instruction. This results in the battery being charged or discharged by the power corresponding to the difference between the travel motor power required for traveling and the power generated by the generator motor. When a delay occurs in the control or response of the engine and generator motor relative to a change in the travel motor power, a large amount of power may be drained from the battery. Further, when the forklift is drastically decelerated from a high speed state, the travel motor power suddenly changes and generates a large regenerative power. In this case, a large amount of power is returned to the battery.
However, the amount of power that may be charged to the battery (tolerable charging power) is limited in accordance with the state of the battery. This limit must not be exceeded when vehicle speed control is executed. For example, when vehicle acceleration is controlled so that the power charged to or discharged from the battery does not exceed the limit, the power supplied to the travel motor may be restricted to slow the acceleration. However, when the vehicle is decelerating, the vehicle must be stopped when the driver intends to do so. Thus, the power supplied to the travel motor cannot be restricted so that the power charged to or discharged from the battery does not exceed the limit.
Japanese Laid-Open Patent Publication No. 2009-40211 discloses a hybrid forklift that prevents the power of the battery from exceeding a set value, such as a rated value, subsequent to charging while maintaining the desired vehicle deceleration rate. When the forklift detects at least one of a plurality of operations being performed, namely, the brake being applied, the accelerator being released, and a switch back operation being performed, the forklift shifts to regenerative power control. In this case, the forklift determines whether or not there is a need to perform a process for reducing the power charged to the battery. When determining that such a reduction process must be performed, the forklift executes at least one of the following processes:
(1) set an engine output command value to 0;
(2) perform fuel cut-off; and
(3) hold the generator motor at the present rotation speed with power stored in the battery or drive the generator motor with power stored in the battery so that the rotation speed decelerates at a rate smaller than a predetermined deceleration rate.
When determining that the reduction process no longer needs to be performed, the forklift ends the reduction process and returns to normal control.
In the forklift of the above publication, the process for reducing the power charged to the battery during the regenerative power control prevents the battery from being over-charged. However, in the reduction process, consideration is not made to controlling the power generated by the generator motor in accordance with a target value. This is because the generator motor generates power from the torque of the inertial rotation it produces when setting the engine output command value to 0 or when performing fuel cut-off. Thus, when cargo handling is required, the necessary amount of rotation produced by the pump may not be obtained. As a result, to obtain the required pump rotation amount when cargo handling is required, the generator motor uses the power of the battery to produce rotation. This may consume power in an unnecessary manner. Further, in the reduction process for forklift of the above publication, consideration is not made to whether or not the power charged to the battery when the travel motor is driven for regenerative power generation exceeds the maximum power tolerated for charging (tolerable charging power). When the forklift carries cargoes, the weight of the vehicle may become 1.5 to 2 times greater than when it is not carrying any cargoes. A significant change in the vehicle weight significantly changes the regenerative power of the travel motor and significantly changes the power charged to the battery. Thus, the sum of the power charged to the battery during regeneration (i.e., regenerative power of the travel motor) and the power generated by the generator motor must be controlled so as not to exceed the tolerable charging power of the battery.